Printed circuit board assemblies are ubiquitously employed in a wide range of products ranging from mass scale consumer products to sophisticated instruments used for specialized applications. It is highly desirable that printed circuit board assemblies, particularly ones that are used in mass scale consumer products, are manufactured in a cost-effective manner and are highly reliable to operate. Towards this end, most manufacturers of printed circuit board assemblies use automated machinery that execute various functions such as picking and placing components upon a printed circuit board, followed by soldering these components upon the printed circuit board. The completed printed circuit board assembly is then inspected for quality by various types of inspection machines that carry out the inspection in a very rapid and efficient manner. One of these inspection machines is known in the industry as a bed-of-nails tester. The bed-of-nails tester is used to test the integrity of various soldered connections as well as to detect certain types of failures in the components mounted upon the printed circuit board.
The types of testing performed by the bed-of-nails tester is traditionally referred to as in-circuit testing and includes various functional tests to evaluate various circuit functions of the printed circuit board assembly and continuity tests that are used to check for abnormal connections such as short circuits and/or open circuits. A short circuit condition can occur for example, when an excessive amount of solder causes a solder bridge to be formed between two solder pads on the printed circuit board assembly. Abnormal open circuit conditions can occur for example, when no solder is applied to a solder pad, or when an inadequate amount of heat is applied for melting the solder on to the solder pad.
A typical bed-of-nails tester includes various fixtures and mounting elements to ensure reliable contact between spring-loaded probes and solder pads (or test nodes) located on a bottom surface and/or a top surface of a printed circuit board assembly that is to be tested. The spring-loaded probes are typically provided in the form of a set of probes mounted on a probe plate. Each probe assembly includes a spring-loaded probe pin that extends through a sleeve and makes contact against the solder pad or test node on the printed circuit board assembly when the bed-of-nails tester is placed in operation. The sleeve is mounted in a through-hole in the probe plate and may have a square tail that is accessible from below the probe plate. A wire that is wire-wrapped to this square tail is used for conveying electrical signals between the bed-of-nails tester and a test instrument such as a continuity tester or a signal generator for performing a functional test upon the printed circuit board assembly.
A printed circuit board assembly may contain a large number of solder pads on a bottom surface and it is feasible that the bed of nails tester be provided with an equal number of probes. However, this is generally undesirable because costs associated with using all the solder pads on the bottom surface of a printed circuit board assembly for test purposes, irrespective of the relevance of individual solder pads to the functionality of the printed circuit board assembly being tested, can be prohibitive and unnecessary. Consequently, a traditional test procedure involves identifying a first set of solder pads to be accessed for performing a first type of test, such as a continuity test, upon the printed circuit board assembly. A first probe plate having a first set of probes is accordingly provided for carrying out the continuity test. A second set of solder pads may then be identified for performing a second type of test, such as a functionality test of the printed circuit board assembly. The first probe plate is removed and replaced by a second probe plate having a second set of probes for carrying out the functionality test. An undesirable cost penalty in terms of duplication and redundancy may be incurred as a result of duplicating various elements of the first probe plate in the second probe plate.
Such costs are exacerbated if the test procedure further involves performing additional tests upon the printed circuit board assembly, such as additional functionality tests. Each of these additional functionality tests involves an entire replacement of one probe plate that is customized for carrying out one type of functionality test with another probe plate that is customized for carrying out another type of functionality test.
The traditional test procedure also suffers from a lack of flexibility in terms of allowing changes to be carried out in the tests conducted upon the printed circuit board assembly. For example, a change in a functionality test may involve modifying the number (and locations) of the solder pads used for carrying out the changed functionality test. Accordingly, the probe plate used previously for carrying out the test has to be either discarded or modified in order to match the modified set of solder pads. The replacement process (or modification process) is not only time consuming, which adds to the overall cost of testing the printed circuit board assembly, but also introduces other costs such as design cost, labor costs, manufacturing costs, and wastage costs.